In recent years, color image reading apparatuses (to be referred to as color scanners hereinafter) using CCDs or the like as photoelectric conversion elements, and color printers have been improved in performance. This increases the risk of copying and forging copying-prohibited originals such as bank notes or securities by reading copying-prohibited originals such as bank notes or securities as image data by a color image reading apparatus and outputting them to a color printer. To prevent the forgery, a color copying machine as a combination of a color scanner and color printer often incorporates an anti-forgery apparatus which detects a copying-prohibited original to prohibit its copying.
In addition, a so-called digital watermark technique of embedding information in an image so as not to recognize it with a naked eye is put into practice.